chore: Phase 2 - Archive build.yml workflow

- build.yml → .gitea/workflows/.archived/build.yml.archived
- Reason: GitHub Release action incompatible with Gitea
- Replaced by: merge-to-main.yml (new unified pipeline)
- Status: Gitea will no longer trigger archived workflows

Impact:
- Reduces workflow count from 12 to 11 active workflows
- No duplicate builds on push to main
- New merge-to-main.yml handles all stages (Tier 1-5)

Next: Phase 3 - Validator grouping in ci.yml

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

.gitea/workflows/.archived/build.yml.archived

@@ -1,10 +1,10 @@
-name: Build & Package
+name: ARCHIVED - Build & Package (replaced by merge-to-main.yml)
 
+# This workflow has been replaced by .gitea/workflows/merge-to-main.yml
+# Triggers are disabled. Kept for reference only.
 on:
-  push:
-    branches:
-      - main
-  workflow_dispatch:
+  # DISABLED - use merge-to-main.yml instead
+  workflow_dispatch: ~
 
 env:
   DOTNET_VERSION: '10.0.x'